Understanding Motion and Speed in Physics

Prior Physics Knowledge Review WHAT DO YOU REMEMBER FROM MIDDLE SCHOOL SCIENCE CLASS

Speed How is a frame of reference used to describe motion? What is the difference between speed and velocity? What do you need to know to find the speed of an object? How can you study speed by using graphs?

Observing Motion How is a frame of reference used to describe motion? When an object changes position with respect to a frame of reference, the object is in motion. motion: an object s change in position relative to a reference point frame of reference: a system for specifying the precise location of objects in space and time

Observing Motion, continued Distance measures the path taken. Displacement is the change of an object s position. always includes direction In the diagram: yellow line = distance black arrow = displacement

Speed and Velocity What is the difference between speed and velocity? Speed tells us how fast an object moves, and velocity tells us both the speed and the direction that the object moves. speed: the distance traveled divided by the time interval during which the motion occurred velocity: the speed of an object in a particular direction

Speed and Velocity, continued Velocity is described relative to a reference point. Direction is described as positive or negative along the line of motion. By convention, up and right are usually positive, and left and down are negative. Combined velocities determine the resultant velocity.

Calculating Speed What do you need to know to find the speed of an object? To calculate speed, you must measure two quantities: the distance traveled and the time it took to travel that distance.

Calculating Speed, continued Average speed is calculated as distance divided by time. distance speed = tim e d t = , or v SI unit for speed: meters per second (m/s) constant speed: equal distances in equal amounts of time instantaneous speed: thespeed at a given time

Math Skills Velocity Metal stakes are sometimes placed in glaciers to help measure a glacier s movement. For several days in 1936, Alaska s Black Rapids glacier surged as swiftly as 89 meters per day down the valley. Find the glacier s velocity in m/s. Remember to include direction. 1. List the given and the unknown values. Given: time,t = 1 day distance,d = 89 m down the valley Unknown: velocity, v = ? (m/s and direction)

Graphing Motion, continued Motion can be studied using a distance vs. time graph. time (x-axis) = independent variable distance (y-axis) = dependent variable The slope of a distance vs. time graph equals speed.

Acceleration What changes when an object accelerates? How do you calculate the acceleration of an object moving in a straight line? How can a graph be used to find acceleration?

Acceleration and Motion What changes when an object accelerates? When an object undergoes acceleration, its velocity changes. acceleration: the rate at which velocity changes over time; an object accelerates if its speed, direction, or both change

Acceleration and Motion, continued Acceleration can be a change in speed. An increase or decrease in speed is an acceleration. Acceleration can also be a change in direction. A motorcyclist who rides around the inside of a large barrel is constantly accelerating. A person riding a Ferris wheel at an amusement park is accelerating. The acceleration that occurs in circular motion is known as centripetal acceleration.

Calculating Acceleration How do you calculate the acceleration of an object moving in a straight line? The average acceleration over a given time interval can be calculated by dividing the change in the object s velocity by the time over which the change occurs. ??????? ???????????? =????? ???????? ?????? ???????? ???? ?? ?? ? = ? ? = ?

Math Skills Acceleration A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot. 1. List the given and the unknown values. Given: time,t = 1.5 s initial velocity,vi = 0 m/s final velocity,vf = 14.7 m/s down Unknown: acceleration, a = ? (m/s2 and direction)

Graphing Accelerated Motion, continued Acceleration can also be seen on a distance vs. time graph. The distance vs. time graph is not a straight line when the velocity is not constant. This curved line indicates that the object is under acceleration.

Force What do scientists identify as the fundamental forces of nature? What happens when there is a net force acting on an object? What force always opposes motion? Why is friction sometimes necessary?

Fundamental Forces What do scientists identify as the fundamental forces of nature? These forces are the force of gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. The strong and weak nuclear forces act only over a short distance. The force of gravity is a force that you feel every day. Other everyday forces, such as friction, are a result of the electromagnetic force.

Fundamental Forces, continued Fundamental forces vary in strength. The fundamental forces vary widely in strength and the distance over which they act. Forces can act through contact or at a distance. Pushes and pulls are examples of contact forces. Field forces (like the force of gravity) do not require that the objects touch each other. Both contact and field forces can cause an object to move or to stop moving.

Balanced and Unbalanced Forces What happens when there is a net force acting on an object? Whenever there is a net force acting on an object, the object accelerates in the direction of the net force. net force: the combination of all forces acting on an object

Balanced and Unbalanced Forces, continued Balanced forces do not change motion. Forces are balanced when the net force is zero. Example: For a light hanging from the ceiling (at rest), the upward force due to tension in the cord balances the downward force of gravity.

Balanced and Unbalanced Forces, continued Unbalanced forces do not cancel completely. Forces are unbalanced when the net force is greater than zero. The object will accelerate in the direction of the net force. Example: If you push a box to the east and your friend pushes the box to the north, the box will accelerate in a northeasterly direction.

The Force of Friction What force always opposes motion? The force of friction always opposes the motion. friction: a force that opposes motion between two surfaces that are in contact

Friction and Motion Why is friction sometimes necessary? Friction is necessary for many everyday tasks to work correctly.

Friction and Motion, continued Unwanted friction can be lowered. using low-friction materials, such as nonstick coatings on cooking pans using lubricants, such as motor oil, wax, and grease Helpful friction can be increased. scattering sand on icy roads to keep cars from skidding wearing textured batting gloves when playing baseball to make it easier to grip the bat

Friction and Motion, continued Cars could not move without friction. Without friction between the tires and the road, the tires would not be able to push against the road and the car would not move forward. The force pushing the car forward must be greater than the force of friction that opposes the car s motion. Because of friction, a constant force must be applied to a car just to keep it moving at the same speed.

Friction and Motion, continued